1. Field of the Invention
The present invention relates to a three-terminal organic electro-luminescent (EL) device having organic and/or polymeric materials as the luminescent semi-conductive species with a third electrode besides an anode and a cathode and, more particularly, to a three-terminal organic EL device resulting in light-emitting triode (or transistor) structures with increased efficiency, decreased turn-on voltage, and increased brightness, thereby easily controlling a luminance of a pixel in the organic EL device.
2. Description of the Related Art
Considerable progress has been made in organic electro-luminescent (EL) diodes utilizing organic and/or polymeric materials as the luminescent semi-conductive species ever since Tang and VanSlyke demonstrated efficient electro-luminescence from organic molecular materials in 1987 (C. W. Tang and S. A. VanSyke, Appl. Phys. Lett. 51, 913 (1987)). Their EL diode devices emphasized a bilayer structure in which an additional hole transport layer of an aromatic diamine was provided to balance the transport of electrons in the light-emitting layer of tris (8-quinolinolato) aluminum (Alq3), resulting in high efficiency and luminance more than 1000 cd/m2 at low drive voltages less than 10V DC. Remarkable progress in a green EL diode has been developed so that an initial luminance of 300 cd/m2 and a half decay time of more than 10000 hours are achieved. Recently, research and development in the area of the organic EL diode device has been extending its focus, not only to the emitting mechanism and materials for obtaining high efficiency, long lifetime, and full-color emissions, but also to the peripheral technologies in search for flat panel display application.
Parallel to developments made with low-weight molecular film devices, there have been constant efforts to discover macromolecular polymeric materials for polymeric EL diode devices and to understand the mechanism of device operation (J. H. Burroughes, D. D. C. Bradly, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, Nature 347, 539 (1990)). Conjugated polymers, such as poly (phenylene-vinylene) (PPV) and its derivatives with high luminescence efficiency, have enormous advantages when used for fabricating EL diode device, combining the good charge transport property with the desirable structural properties of polymers. Studies of the charge transport in PPV reveal that the bulk conduction properties, i.e., carrier mobilities, play an important role in the device characteristics. Nonconjugated polymers have also been shown to be useful in fabricating EL diode. For example, although poly (N-vinylcarbazole) (PVK) is not electronically conductive, it shows photoconductivity and has relatively high hole drift mobilities due to its carbazole side groups.
For details on recent progress in various organic EL diode devices, see examples, the followings:
C. W. Tang and S. A. Van Syke, Appl. Phys. Lett. 51, 913 (1987); C. W. Tang, S. A. Van Syke, and C. H. Chen, J. Appl. Phys. 65, 3610 (1989); C. Adachi, S. Tokito, T. Tetsui, and S. Saito, Appl. Phys. Lett. 55, 1489 (1989); C. Adachi, S. Tokito, T. Tetsui, and S. Saito, Appl. Phys. Lett. 56, 799 (1989); J. H. Burroughes, D. D. C. Bradly, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, Nature 347, 539 (1990); D. Braun and A. J. Heeger, Appl. Phys. Left. 58, 1982 (1991); W. Tachelet, S. Jacobs, H. Ndayikengurukiye, H. J. Geise, and J. Gruner, Appl. Phys. Left. 64, 2364 (1994); J. Kido, K. Hongawa, K. Okuyama, and K. Nagai, Appl. Phys. Left. 63, 2627 (1993); G. Gustaffson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A. J. Heeger, Nature (London) 357, 477 (1992); Y. -E. Kim, H. Park, and J. -J. Kim, Appl. Phys. Lett. 69, 599 (1996); H. H. Kim, T. M. Miller, E. H. Westerwick, Y. O. Kim, W. Kwock, M. D. Morris, and M. Cerullo, J. Lightwave Technol. 12, 2107(1994); F. Li, H. Tang, J. Anderegg, and J. Shinar, Appl. Phys. Lett. 70, 1233 (1997); and L. S. Hung, C. W. Tang, and M. G. Mason, Appl. Phys. Lett. 70, 152 (1997).
The organic EL diode device has not only a fast response time of several microseconds but also an excellent brightness of self-emission with wide viewing angle under a low driving voltage. In particular, the organic EL device can be easily manufactured in a thin film and a flexible form. Also, the organic EL device has an advantage to be able to emit light in a wide visible spectral range. Thus, due to their wide viewing angle, bright self-emission, and ease of color tunability and processability, displays based on the organic EL diode such as transparent organic EL diode, flexible organic EL diode, stacked organic EL diode, microcavity organic EL diode, and inverted organic EL diode have become promising candidates for a flat panel display instead of a liquid crystal display (LCD), a plasma display panel (PDP) or a field emission display (FED).
Cross sectional views of a conventional organic EL diode device were shown in FIGS. 1a and 1b. As shown in FIG. 1a, a first electrode 11 serving as an anode (A), composed of Indium Tin Oxide (ITO), polyaniline or Ag having large work function, is formed on a substrate 10. At least one first organic material layer 22 composed of low molecular weight organic compounds and/or polymer materials is formed on the first electrode 11. A second organic material layer 26 composed of another low molecular weight organic compounds and/or polymer materials is formed on the first organic material layer 22. And at least one light-emitting layer is included in either the first organic material layer 22 or the second organic material layer 26, or interposed therebetween. A second electrode 16 serving as a cathode (C), composed of Al, Mg, Li, Ca, or complex compounds thereof having low work function is formed on the second organic material layer 26, while opposing the first electrode 11.
The first organic material layer 22 and the second organic material layer 26, as shown in FIG. 1b, may include a hole injection layer 12 for injecting holes, a hole transporting layer 13 for conveying holes, an organic light-emitting layer 14 and an electron transporting layer 15 for injecting and conveying the electrons, which are sequentially formed. Here, the electron transporting layer 15 may include a hole blocking layer and an electron injection layer.
When a voltage is applied between the first electrode 11 and the second electrode 16 of the organic EL diode device shown in FIGS. 1a and 1b, the holes injected from the first electrode 11 migrate through the hole injection layer 12 and the hole transporting layer 13 into the organic light-emitting layer 14, and the electrons injected from the second electrode 16 migrate into the organic light-emitting layer 14 through the electron transporting layer 15. Thereafter the recombination of the injected holes and electrons at the organic light-emitting layer 14 excites the emitting centers, thereby emitting EL lights and displaying images. This radiative recombination luminance is directly proportional to the concentrations of charge carriers and the electron-hole radiative recombination probability at the organic light-emitting layer 14.
With reference to potential diagrams for energy band structure shown in FIG. 1c, a radiation principle of the organic EL diode can be explained. The electron is symbolized as a circled “−” (⊖), the hole is symbolized as a circled “+” (⊕) and migration of the electron and hole is represented as an arrow. The reference numerals 11, 12, 13, 14, 15, and 16 appeared in FIG. 1c mean energy potentials of the anode (A) 11, the hole injection layer (HIL) 12, the hole transporting layer (HTL) 13, the organic light-emitting layer (EML) 14, the electron transporting layer (ETL) 15, and the cathode (C) 16, respectively. Also, φA and φC indicate work functions of the anode (A) 11 and the cathode 16 (C), respectively, while EA and IP indicate the electron affinity and the ionization potential of each organic layer, respectively. HOMO and LUMO represent the highest occupied molecular orbital (valance band) and lowest unoccupied molecular orbital (conduction band) of the organic layers.
First, when a voltage (VCA) is not applied between the anode (A) 11 and the cathode (C) 16, the hole injection layer 12, the hole transporting layer 13, the organic light-emitting layer 14 and the electron transporting layer 15 are in thermodynamic equilibrium state so that Fermi levels thereof are identical to each other, as shown in D1 of FIG. 1c. When the voltage VCA is applied between the anode (A) 11 and the cathode (C) 16, the holes are gradually injected from the anode (A) 11 into the highest occupied molecular orbital (HOMO) state of the hole injection layer 12 and the electrons are gradually injected from the cathode (C) 16 into the lowest unoccupied molecular orbital (LUMO) state of the electron transport layer 15. Here, if the applied voltage VCA is less than a turn-on voltage (VONSET), then the holes and the electrons cannot migrate to the organic light-emitting layer 14 and there is no occurrence of electro-luminescence, as shown in D2 of FIG. 1c. However, if the applied voltage VCA is exceeds the turn-on voltage VONSET, then the holes and the electrons can migrate into the organic light-emitting layer 14 through the HIL (12), HTL (13), and ETL (15). And electro-luminescence can be generated from the radiative recombination of the holes and the electrons (exciton) in EML (14) (S1−>S0+hν), as shown in D3 of FIG. 1c. In this case, the current flows of anode (IA) and cathode (IC) can be written as IA=IC. Thus, the electro-luminance is directly proportional to the electron-hole radiative recombination probability and concentrations of injected electrons and holes at the organic light-emitting layer 14.
One of techniques for controlling a luminance in the conventional two-terminal organic EL diode device was disclosed in Korean Laid-Open publication 2001-14600. FIG. 2 illustrates an active matrix type organic EL display having red (R), green (G), and blue (B) pixels disclosed in Korean Laid-Open publication 2001-14600 and FIG. 3 illustrates a voltage adjusting circuit for controlling the luminance of the pixels.
Referring to FIG. 2, each pixel has an anode (A) 11, R, G, and B light-emitting layers 14, and a cathode (C) 16. The anode (A) 11 is connected to a source electrode of a polysilicon thin film, which forms a thin film transistor (TFT) 40 together with a drain electrode of the polysilicon thin film 41 and a gate electrode 39. The drain electrode is connected to a power supply terminal 32. A power supply voltage VDD for driving the organic EL display is connected to the power supply terminal 32 and the cathode (C) 16 is grounded. When the gate electrode 39 is turned on, the power supply voltage VDD is supplied to the anode (A) 11 and the pixels emit light by the voltage between the anode (A) 11 and the cathode (C) 16. The undescribed reference numerals 44 and 10 represent an interlayer dielectric and a substrate, respectively.
The voltage adjusting circuit in FIG. 3 controls the power supply voltage VDD applied to the anode (A) 11 according to the current amount introduced to the cathode (C) 16. If many pixels emit EL light, for example all of R, G, and B pixels in FIG. 2 radiate, the current amount flowing into the cathode (C) 16 is increased. Thereby, voltage V1 in a current detection circuit 52 rises and an output voltage V2 of an inverted-voltage amplification circuit 54 descends. The descended voltage V2 is amplified at an amplification circuit 56 and in turn the amplified voltage is supplied to the power supply terminal 32. On the contrary, if few pixels emit EL light, for example only one of R, G, and B pixels in FIG. 2 radiate, the current amount flowing into the cathode (C) 16 is decreased. Thereby, the voltage V1 in a current detection circuit 52 descends and the output voltage V2 of the inverted-voltage amplification circuit 54 increases. Therefore the high voltage VDD is supplied to the power supply terminal 32.
That is, the voltage adjusting circuit in FIG. 3 reduces a luminance of each pixel and thus degrades luminance of the organic electro-luminescent display in case where many pixels emit EL light, and enhances the luminance of the organic EL display and degrades the luminance thereof in case where few pixels radiate.
The adjustment of the luminance using the circuit in FIG. 3 needs an accurate and precise control for the current and voltage between the anode 11 and the cathode 16. Thus, the method for adjusting the luminance described above, is more complicate than that employed in another flat panel display such as a liquid crystal display (LCD).
Beside of the complexity for adjusting the luminance, the organic EL diode devices suffer three important drawbacks that impede large-scale applications:    (1) Emission of light at the desired brightness levels often requires the application of a relatively high voltage.    (2) The external conversion efficiency is low.    (3) Brightness is limited, particularly at voltage below 5 volts.
Therefore, general and broad needs still exist for device concepts that result in light-emitting structure with increased efficiency, decreased turn-on voltage, and increased brightness.